Iodine-Silver Trifluoroacetate

I2-CF3CO2Ag
(I2)

[7553-56-2]  · I2  · Iodine-Silver Trifluoroacetate  · (MW 253.80) (CF3CO2Ag)

[2966-50-9]  · C2AgF3O2  · Iodine-Silver Trifluoroacetate  · (MW 220.89)

(a source of positive iodine for iodination of aromatic compounds and for oxidation of alkenes)

Physical Data: CF3CO2Ag: mp 257-269 °C (dec).

Solubility: CF3CO2Ag: sol benzene, diethyl ether, water.

Form Supplied in: white solid; widely available.

Analysis of Reagent Purity: contents of Ag can be assayed conveniently by volumetric titration of AgI.

Preparative Method: extraction of an aqueous solution of AgNO3 and CF3CO2Na with ether.1

Handling, Storage, and Precautions: should be protected from light.

Introduction.

The combination of Iodine and Silver(I) Trifluoroacetate, usually in a chlorinated solvent, is believed to involve the formation of a complex between silver trifluoroacetate and electrophilic iodine trifluoroacetate.1

Aromatic Iodination.

The reagent can iodinate various aromatic compounds, the conditions being dependent upon the reactivity of the substrate. For example, benzoic acid was iodinated in nitrobenzene at 150 °C to give 3-iodobenzoic acid in 84% yield.2 On the other hand, iodination of veratrole is performed in refluxing chloroform (eq 1).3

An elegant application of this mild aromatic iodination procedure was reported by Carruthers and co-workers (eq 2).4

Oxidation of Alcohols and Ketones.

Primary alcohols can be oxidized to the corresponding aldehydes in low yields. Secondary alcohols are oxidized to the corresponding ketones, but the reaction is complicated by further oxidation to the 1,2-dicarbonyl compounds. Ketones are transformed into the corresponding 1,2-dicarbonyl compounds, e.g. biacetyl can be isolated in 50% yield from 2-butanone. These reactions are usually not of synthetic value but, in cases where such functional groups are present, care should be taken to prevent their oxidation.5

Oxidation of Alkenes.

The positively charged iodine reacts with electron-rich alkenes by analogy with the combination Iodine-Silver Acetate. Depending on the structure of the substrate, the intermediate iodonium ion can undergo intra- or intermolecular consecutive reactions. The simplest case is represented by the treatment of 5a-cholest-2-ene with I2-CF3CO2Ag, where an intermolecular iodotrifluoracetoxylation was observed (eq 3).6

cis-Vinylsilanes behave similarly, i.e. on reaction with I2-CF3CO2Ag, the corresponding 1,2-adduct can be isolated. On elimination with Potassium Fluoride, trans-vinyl iodides are formed (eq 4). However, addition of only iodine to the same cis-vinylsilane gave the corresponding cis-vinyl iodide. Other halogens (Cl2, Br2) gave the 1,2-adduct, which can be converted into the corresponding trans-vinyl halide by treatment with Sodium Methoxide.7

In a more elaborate approach, tetrahydrofurans were prepared in a stereoselective manner from the corresponding homoallylic alcohols by treatment with I2-CF3CO2Ag (eqs 5-8).8 From a stereochemical point of view it is interesting to note that the products observed are the results of syn addition across the double bond, which is contrary to the expected direction of addition. On the other hand, Benzeneselenenyl Chloride adds to form the corresponding 2,5-trans-tetrahydrofurans, i.e. the result of an anti addition. Both (E)- and (Z)-alkenes behave similarly, i.e. syn addition with I2-CF3CO2Ag and anti addition with PhSeCl. Another unexplained observation is that the direction of the nucleophilic attack is dependent upon the stereochemistry of the alkene, i.e. (E)-alkenes are attacked by iodine on the most hindered side (eqs 5 and 7), whereas (Z)-alkenes are attacked on the least hindered side (eqs 6 and 8).

The use of acetonitrile as the solvent is critical for the success of these reactions.

Related Reagents.

Iodine-Silver Benzoate.


1. Haszeldine, R. N. JCS 1951, 584.
2. Haszeldine, R. N.; Sharpe, A. G. JCS 1952, 993.
3. Janssen, D. E.; Wilson, C. V. OSC 1963, 4, 547.
4. Carruthers, W.; Coggins, P.; Weston, J. B. JCS(P1) 1991, 611.
5. Bergmann, E. D.; Shakah, I. JCS 1959, 1418.
6. Hey, D. G.; Meakins, G. D.; Pemberton, M. W. JCS(C) 1966, 1331.
7. Miller, R. B.; Reichenbach, T. TL 1974, 543.
8. Lipshutz, B. H.; Barton, J. C. JACS 1992, 114, 1084.

Lars-G. Wistrand

Nycomed Innovation, Malmö, Sweden



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